This invention relates to a method of fabricating a semiconductor device, and more particularly to a fabricating method capable of improving the electric characteristics of a gate electrode lowered in resistance being composed of polycrystalline conductive film and low-resistance conductive film such as an MOS type integrated circuit, and preventing insulation breakdown of the gate insulating film.
To obtain fine and high performance transistors in a field effect type semiconductor integrated circuit, it is a general practice to lower the resistance of the gate electrode by forming a low resistance conductive film on the gate electrode composed of polycrystalline conductive film and heating it. For example, as a low resistance conductor, a high melting point metal or an alloy thereof with silicon, that is, silicide is deposited on a polycrystalline silicon used as gate electrode, and is heated to about 500.degree. C., so that the high melting point metal or silicide easily reacts with polycrystalline silicon to be entirely transformed into a silicide, thereby lowering the resistance. The sheet resistance of thus formed conductive film is as low as about 1 to 10 .OMEGA./square, and high speed operation of the semiconductor integrated circuit is expected. On the other hand, depending on the kind of metal, it may react with materials than polycrystalline silicon, and in particular since titanium reacts also with a silicon oxide film used as gate electrode material, although it is very low in resistance, it is difficult to use such conductive film as a gate electrode. Hence, tungsten and molybdenum which are slightly higher in resistance than titanium, but hardly react with a silicon oxide film, are generally used. The reactivity and reaction product generating energy between this high melting point metal or silicide with a silicon nitride film or a silicon oxide film are described in Chapter 2 of volume 9 of VLSI Electronics: Microstructure, by N.G. Einsprach (Academic Press). Meanwhile, ease of reaction can be judged by the magnitude of the activation energy, and it is known that the reaction is easier when the activation energy is smaller.
By heat treatment after forming a low resistance conductive film on a gate electrode composed of polycrystalline conductive film, mutual constituent atoms move at the interface between the polycrystalline conductive film and low resistance conductive film, and the vicinity of the interface is alloyed and lowered in resistance. However, in the interface between the crystal grains of the polycrystalline conductor, that is, in the grain boundary, since the bonding force of constituent atoms of the polycrystalline conductor is weak, replacement of constituent atoms of low resistance conductor film with atoms is promoted faster than in the crystal grains, and the low resistance conductor tends to be diffused locally along the grain boundary. When the low resistance conductor is diffused locally and tears the insulation film beneath the gate electrode, the gate electrode is short-circuited with other electrode to lower the reliability of the transistors or the yield of production. To prevent this, a method of depositing an ultrathin silicon nitride film between the polycrystalline conductor and low resistance conductor by CVD (chemical vapor deposition) to inhibit the local diffusion of low resistance conductor and enhance reliability is mentioned in the IEEE Transactions of Electron Devices, Vol., ED-33, No. 4, 1986, pp. 464-467. It is however, difficult to deposit an ultrathin silicon nitride film by CVD at high reproducibility of film thickness, and, still more, it is necessary to deposit at relatively low temperature to prevent change in the electric characteristics due to re-diffusion of impurities in the impurity diffusion layer. Accordingly, it has been difficult to obtain a dense film properties sufficient to inhibit the local diffusion of the low resistance conductor, and improvement of reliability of yield has been inadequate.